1. Field of the Invention
The present disclosure relates to a method of manufacturing a rotor core (pole core) having a plurality of claws of an electric rotating machine.
2. Description of Related Art
It is known to use, as a rotor of an electric rotating machine such as a vehicle-use alternator, a pair of pole cores each having a cylindrical boss section formed with an axial hole thereinside, a disk section radially extending from one end of the boss section, and a plurality of claw sections axially extending from the periphery of the disk section at equal intervals toward the other end of the disk section. Each adjacent two of the claw sections form a V-shaped channel reaching the outer periphery of the disk section.
The pair of the pole cores are disposed opposite to each other such that the claw sections of one pole core are located in the V-shaped channels of the other pole core with a certain clearance therebetween. A field coil is disposed between the disk sections and the claw sections, and a shaft is pressure-inserted into the axial bores to form a Lundell-type rotor core.
Such a pole core is manufactured by cold forging or hot forging in view of productivity, as shown, for example, in Japanese Patent No. 3609745. This patent document describes a method of manufacturing such a pole core as shown in FIG. 8, in which a boss section 2a is placed downward, an axial hole 2b is punched in the boss section 2a using a punch p of an upper mold m3a and a center die d of a lower mold m3b, and simultaneously with this, claw sections 4a are ironed to stretch while being bent until they form an angle of 90 degrees with a disk section 90 so that step portions 4a1 of the claw sections 4a are pressed against step portions f2 of the lower mold m3b and shaped into claw step portions 4b. 
However, in this forging method, since the outer surfaces of the connecting portions between the claw sections 41 and the disk section 3a are extended while the claw sections 4a are bent and the axial hole 2b is formed, the surfaces of the disk section 3a are extended axially inward in the vicinity of the axial hole. As a result, since a residual bending stress forcing the disk section 3a to project outwardly remains, the claw sections 4a are likely to be displaced in the direction to increase the angle of the bend due to springback after the molds are removed. Further, since the diameter of the axial hole 2b increases on the side of the surface of the disk section 3a after the pole core is removed from the molds, the axial hole 2b is likely to have a tapered shape due to the difference between the diameter on the side of the surface of the disk section 3a and the diameter on the side of the end surface of the boss section 2a. In addition, the axial hole 2b is sags at its open end due to a tensile stress when the axial hole 2b is punched by the punch p.
As a result, the interference between the shaft and the axial hole 2b may become excessively large on the side of the end surface of the boss section 2a. In this case, it become difficult to pressure-insert the shaft into the axial hole 2b. 
Further, when the interference is excessively small on the side of the disk section 3a, the fastening force between the shaft and the axial hole 2b occurs only at side of the disk section 3a, and the fastening force becomes inadequate. As a result, since the pole core easily vibrates with respect to the shaft due to deviation of the shaft from the rotating axis, magnetic attraction power and vibration of use environment, there occur adverse effects such as occurrence of magnetic nois, reduction of output power and reduction of lifetime of the rotor.